In recent years, a magnetic head for perpendicular magnetic recording has been suggested to realize high recording density, large capacity or miniaturization of a magnetic disk device as a disk device. In this type of magnetic head, a recording head includes a main magnetic pole which produces a perpendicular magnetic field, a write shield magnetic pole provided on the trailing side of the main magnetic pole across an intervening write gap, and a coil for supplying a magnetic flux to the main magnetic pole. Further, there is suggested a high-frequency (microwave) assisted head wherein a microwave assisted element such as a spin-torque oscillator is provided in the write gap between the medium-side end portion of the write shield magnetic pole and the main magnetic pole. Current is supplied to the spin-torque oscillator through the main magnetic pole and the write shield magnetic pole.
In the high-frequency assisted head, the spin injection layer and the oscillation layer of the high-frequency assisted element are allocated in the write gap. Each of the spin injection layer and the oscillation layer has a constant film thickness from the air bearing surface (ABS) side of the head to a position away from the ABS in a perpendicular direction (in other words, a deep position). Normally, in the write gap, the head travel direction component of the gap magnetic field is stronger in a position away from the ABS in a perpendicular direction (in other words, a deep position) than near the ABS. Thus, the magnetic field is not uniform in the height direction of the write gap. The magnetization of the spin injection layer easily points in the head travel direction in the deep position away from the ABS; however, the magnetization of the spin injection layer easily becomes unstable on the ABS side. The magnetization of the spin injection layer is difficult to point in a certain direction, and thus, the magnetization of the oscillation layer is difficult to uniformly rotate. As a result, good oscillation cannot be obtained. It is difficult to assure energy sufficient to invert the magnetization in the recording layer of the recording medium.